1. Field of the Invention
The present invention relates to a wireless network system and, more particularly to a mobile ad-hoc network system.
2. Description of the Related Art
A mobile ad-hoc network is a temporary network autonomously formed by terminals with mobility by using an air interface and stands alone from an infrastructure network.
The mobile ad-hoc network has been developed to temporarily form a network in an area where an infrastructure network does not exist or development of a network is not easy based on the infrastructure network. Initially, research on the ad-hoc network was conducted for a military purpose, and recently, its application is being extended to various fields of a real life such as a PAN (Personal Area Network).
The ad-hoc network operating with only connected terminals without the help of the infrastructure has many problems such as limited battery capacity of terminals, disconnection of a network connection due to movement of terminals, security, charging, and so on, and in order to solve such problems, various researches are being conducted centering on a MANET (Mobile Ad hoc NETwork) WG of an Internet Engineering Task Force (IETF).
Major application fields of the mobile ad-hoc network can include the construction of a temporary network for an emergency service.
Since the mobile ad-hoc network does not require the infrastructure network, it can be applied for an area where using of communication infrastructure is not available, an area where communication infrastructure does not exist such as a desert or a mountainous area, or an area where installation of the communication infrastructure is not easy. Namely, the ad-hoc network can be temporarily constructed for use in a rescue operation or the like.
In addition, the mobile ad-hoc network can be also applied for a case where information is promptly shared or exchanged between persons in a convention hall or in an exhibition hall. Through the ad-hoc network, in small, two persons can exchange an electronic name card, and in large, a rapid contribution can be made among every participant in a standardization conference or broadcasting of information on products can be made in an exhibition, and so on.
Military application is recognized as one of the most usable application fields of the mobile ad-hoc network. Namely, the mobile ad-hoc network can be used for communication among soldiers and between soldiers and military equipments in a battle field where the communication infrastructure can be hardly used.
WPAN (Wireless Personal Area Network) and home networking are recently introduced application fields of the mobile ad-hoc network. In constructing the PAN, a small-scale network among information communication devices such as PDAS or mobile phones, addition, deletion or interworking with a different PAN are all performed based on the ad-hoc concept.
The mobile ad-hoc concept can be also applied for construction of home networking with information devices at home by using a wireless connection technique.
In addition, the mobile ad-hoc network can be also applied for a sensor network in that sensors are installed in a dangerous area or in an area considered to be difficult to approach and constructed as a mobile ad-hoc network for exchanging information therebetween.
FIG. 1A illustrates the structure of the mobile ad-hoc network and FIG. 1B illustrates the structure of the infrastructure network.
As noted in FIGS. 1A and 1B, the mobile ad-hoc network has a relative concept to the infrastructure network which requires centralized controlling.
FIG. 2 illustrates an independently existing ad-hoc network and FIG. 2B illustrates an ad-hoc network interworking with the infrastructure network.
The mobile ad-hoc network can exist in a stand-alone form over the infrastructure network as shown in FIG. 2A or can exist in a form interworking with the infrastructure network by a gateway.
The mobile ad-hoc network uses an air interface for communication between terminals. Compared to a fixed line interface method, the air interface method has such characteristics that a transmission bandwidth is small. Because the air interface method uses a frequency with a limited band, a transmission rate of data transmitted through the band is limited. Also, since a plurality of terminals share the band, the more the number of terminals increases, the more a transmission bandwidth that each terminal can use in average is reduced. In addition, the air interface method has a limited transmission distance. Since the data transmission rate is lowered as the radio transmission distance gets away, the transmission distance is limited to maintain a suitable transmission rate.
In order to resolve the limitation, the mobile ad-hoc network extends a data transfer coverage by using a multi-hop method.
Terminals constituting the mobile ad-hoc network have mobility, so a topology of the network is dynamically changed over time. The terminal mobility includes entering of a new terminal into a network, movement of a terminal in a network and movement of the terminal out of the network, and power ON/OFF of the terminal in the network affects a change of a topology.
Each terminal has a limited radio transmission distance. Thus, as terminals are moved, a set of neighboring terminals that each terminal can directly communicate is also changed. Each terminal broadcasts its existence periodically and maintains information of neighboring terminals that each terminal can directly communicate. And each terminal updates route information according to information of a neighboring terminal. The route information is generated and managed by a routing protocol and the routing protocol can be divided into a proactive method and a reactive method.
In the proactive routing protocol, all terminals maintain fresh route information (the latest route information), and when the route information is changed periodically or network topology changes, the route information is broadcast to all terminals. With the latest route information maintaining, the proactive routing protocol can transfer data through an optimal route always without delay when traffic is generated, but a control message for managing the route information assumes massive proportions.
The reactive routing protocol is a method for searching a route at a point where traffic is generated, which solves the problem of the control message overhead of the proactive routing protocol. Generally, the reactive routing protocol processes route information managed by a terminal to be soft. Namely, when traffic is not transferred during a certain time in the route maintaining information, a terminal deletes the corresponding route information from a routing table. In the reactive method, a delay can be made according to route searching. Thus, the reactive method is more focused on minimizing route search time than searching an optimal route.
Most terminals constituting the mobile ad-hoc network use a limited capacity of battery as an energy source in order to support their mobility. According to circumstances, energy can be stably supplied temporarily when a terminal is mounted in a car or energy can be supplied continuously with a fixed unit. The limitation of energy much affects designing of the routing protocol. For example, if a terminal with a low energy level routes much traffic, remaining battery capacity of the terminal would be gradually reduced and the terminal could not use every route going through the terminal in the end. Thus, each terminal should select a route in consideration of its energy state for a stable data transmission. The energy restriction can cause a unidirectional transmission state of a link. Because a wireless transmission distance of a terminal with much energy is long, transmission from the terminal with much (or more) energy to a terminal with little (or less) energy can be possibly made, but the opposite case is not.
The energy restriction can cause a unidirectional problem in communication between two nodes. Because a wireless transmission of a terminal with much energy can reach farther than a terminal with less energy can did, transmission from the terminal with much (or more) energy to a terminal with little (or less) energy can be possibly made, but the opposite case is not.
Referring to security, the mobile ad-hoc network is exposed in a more dangerous condition compared to the general wired networks, because it uses air interface. In addition, in the mobile ad-hoc network, terminals are insecure with each other in their identity, and in case of routing by the multi-hop method, a security problem arises with respect to data by an intermediate terminal. In case of the control message of the routing protocol, because it is broadcast, its security is more serious than traffic.